To meet different requirements, telecommunication networks with different types of modulation and different transmission rates are used.
For stationary telecommunication networks, such as those of European postal administrations, the CCITT recommendations are generally applied, i.e., in Europe, a channel bit rate of 64 kb/s and pulse-code modulation with A-law companding.
For economic reasons, other stationary telecommunication networks are also implemented in accordance with those CCITT recommendations. This applies in particular with regard to the introduction of ISDN for subscribers in such telecommunication networks. Besides the stationary telecommunication networks, however, there are also tactical telecommunication networks. The latter are characterized by great mobility of the subscribers and possible strong interference on the transmission links. Therefore, a channel bit rate of 16 kb/s or 32 kb/s and other modulation types, e.g., delta modulation in different variants, have been established for tactical telecommunication networks. The same applies analogously to the new civil mobile radio network (C-net), which is currently being introduced.
The different channel bit rates and types of modulation cause problems during connections between subscriber facilities of different telecommunication networks, particularly if the speech or data is to be transmitted in encrypted form. In either case, it is not possible to simply convert the signals from one channel bit rate and type of modulation to another and back.
In general, encryption and decryption in a connection between two telecommunication networks can be accomplished by performing a single-channel encryption in each of the two telecommunication networks and a code conversion (transcoding) at the interface between the different telecommunication networks.
One of the disadvantages of this solution is that it does not permit end-to-end encryption, since at the interface between the two different telecommunication networks, the information must be present in plain text so that a code conversion can be performed.
If the code conversion takes place at the exchange, stringent requirements must be placed on crosstalk attenuation between the individual connections which can be met only with great difficulty, if at all. These requirements are reduced if the code conversion is performed in a special facility.